/Engine/Source/Runtime/Core/Public/Internationalization/TextFormatter.h
Incredibly low-level version of format. You should only be calling this if you're implementing a custom argument modifier type that itself needs to format using the private arguments
static [FString](API\Runtime\Core\Containers\FString) Format ( const [FTextFormat](API\Runtime\Core\Internationalization\FTextFormat) & InFmt, const FPrivateTextFormatArguments & InFormatArgs )